1. Technical Field
The invention relates to gallium arsenide based devices-devices including one or more regions of GaAs, AlGaAs, InGaAs. Primary use is in integrated circuits with function dependent upon electronic, photonic or hybrid properties. A significant category of electronic circuits incorporates heterojunction bipolar transistors having n-doped base regions.
2. Description of the Prior Art
It has been decades since the first confident prediction that silicon would yield to compound semiconductors, III-Vs, II-VIs, as well as ternaries and quaternaries, resulting in activity often provoked by the particularly favorable properties of GaAs and variants. (Use of the designation "GaAs" herein is intended to encompass such variants as including e.g. AlGaAs, InGaAs as well as higher order compositions containing sufficient retained Ga and As to be equivalent to the approximately 45 mol % GaAs as required for assurance of direct bandgap and related device properties.) The prediction largely concerned fundamental characteristics, in large measure due to direct bandgap-.apprxeq.1.4 electron volts for the binary material; a value which can be altered to suit device needs by incorporation of other elements in group III or V sites. Operational advantages relative to silicon were correctly appreciated as offering significantly increased speed of operation as well as versatility in operating wavelength extending to photonic.
Failure for such GaAs-based devices to broadly supplant silicon is to large extent based on a now-familiar economic pattern. Significant cost entailed switching to the new material--in equipment design/replacement and in retraining of workers provoked effort to stay with silicon. Intensive worldwide effort has yielded ever-improving silicon devices with realization of properties previously expected to require substitution by GaAs. With respect to most prevalent LSI/VLSI structures, as exemplified by e.g. megabit and larger capacity chips, the likely correct view is to continued dominance of silicon.
Fundamental properties of GaAs, however, suggest substitution for silicon in specialized device classes. Present use in radiation detectors, e.g. in radar detectors, is based on speed--on response time which in present GaAs devices may be of the order of 60 GHz as compared with speeds of 2-4 GHz as offered by similar structures made of silicon. Of course, GaAs offers photonic properties not available by use of silicon. Belated promise of high density integrated laser structures-at this time particularly of Surface Emitting Lasers--increases significance of this aspect.
While GaAs has, to some minor extent, fulfilled such promise, use has been limited by a number of processing and composition limitations.